Long-period gratings are in-fiber devices that can couple light between two dissimilar co-propagating spatial modes in a fiber. These devices typically comprise a length of optical fiber with a long-period grating which is formed by a plurality of refractive index perturbations spaced along the fiber by a periodic distance .LAMBDA.. In general, long-period grating devices are those in which the period is at least 10 times larger than the wavelength of the input light. Typically .LAMBDA. is in the range 15-1500 .mu.m. The modes being coupled by the grating are either different modes guided by the core or are those guided by the cladding (with the next surrounding region, say air or a coating, acting as the effective cladding for these modes).
The major functional difference between long-period gratings and conventional short-period Bragg gratings (periods .LAMBDA.&lt;1 .mu.m) is that there is no backreflected mode in a long-period grating. As an example, the light can be coupled out of the core from a fundamental mode and into the cladding of a fiber on a wavelength selective basis.
In the past, these devices have been used as simple mode-converters (Hill et al., U.S. Pat. No. 5,104,209, Method of Creating an Index Grating in an Optical Fiber and a Mode Converter Using the Index Grating), as wavelength-dependent loss elements in broadband amplifiers (Vengsarker et. al., Opt. Lett. Vol. 21, p. 336, 1996) in high-power fiber lasers, (Grubb and Stentz, Laser Focus World, Feb., 1996, p. 127), and as band-rejection filters (Vengsarkar, et. al., Journal of Lightwave Technology, vol. 14 p. 58, 1996) See also U.S. Pat. No. 5,430,817 issued to A. M. Vengsarkar on Jul. 4, 1995, which is incorporated herein by reference.
While these devices provide an elegant solution to many problems in fiber communications, they exhibit temperature sensitivity. The peak wavelength .lambda..sub.p shifts by 4-10 nm per 100.degree. C. change in temperature. For some applications where ambient temperatures can fluctuate between 5.degree. and 45.degree. C., such variations are not acceptable and temperature compensation schemes are desirable. The simplest solution is to place the grating device in a controlled temperature chamber; this solution, however, adds temperature controllers, thereby adding cost and increasing reliability concerns.
Several other solutions have also been suggested. For example, U.S. patent application Ser. No. 08/695,180, is now patented with U.S. Pat. No. 5,703,978, in the name of DiGiovanni et al. demonstrates a method of changing the fiber composition and profile such that the temperature dependence of the peak wavelength can be reduced below 4 nm per 100.degree. C. change in temperature. This solution, while useful for many applications, places the burden of stabilization on accurate preform and fiber manufacture. It further eliminates the possibility of using standard telecommunication fibers as the grating host, raises the necessity of splicing and introduces added insertion losses.
Another method is described in U.S. Pat. No. 5,757,540, "Long-Period Fiber Grating Devices Packaged for Temperature Stability", issued May 26, 1998, to Judkins et al., and assigned to the present assignee. The method of the '540 patent a specifically tailored grating where the dependence of the peak wavelength is opposite in polarity for strain and temperature. Such a specially designed grating is packaged in a material with a specific thermal expansion and leads to a packaged temperature-insensitive device. This method, while attractive, places the burden of stabilization on the packaging material and, by its operating principle, requires that the fiber grating be placed under strain during its entire lifetime.
In addition, in prior efforts to compensate for temperature instability, the grating portion of the fiber is bare and not coated. The fiber grating is uncoated because once the temperature-insensitivity has been accomplished, the additional step of recoating the grating would change the spectral and thermal properties of the device, thus rendering it unstable to temperature variations. Leaving the grating section uncoated raises issues of reliability and packaging stability over the lifetime of the device. There is therefore a need for a recoated temperature insensitive grating that can use conventional communication grade fiber (which helps lower splice losses) and that does not need to be strained during its lifetime.